os: add fuchsia constraint value

os/BUILD

diff --git a/os/BUILD b/os/BUILD
index e35460a..2c2d281 100644
--- a/os/BUILD
+++ b/os/BUILD
@@ -96,3 +96,8 @@
     name = "wasi",
     constraint_setting = ":os",
 )
+
+constraint_value(
+    name = "fuchsia",
+    constraint_setting = ":os",
+)